A capacitor used in a circuit associated with the central processing unit (CPU) in personal computers and the like is required to have high capacitance and low ESR (equivalent series resistance) in order to prevent the fluctuation of voltage and reduce the generation of heat at the passing of a high ripple current. In general, an aluminum or tantalum solid electrolytic capacitor is used.
The solid electrolytic capacitor is constituted by an aluminum foil having fine pores in the surface layer or a tantalum powder sintered body having fine pores in the inside, which is serving as one electrode (electric conductor), a dielectric layer formed on the surface layer of the electrode, and the other electrode (usually a semiconductor layer) provided on the dielectric layer.
The ratio of the semiconductor layer formed on the dielectric layer is indicated by impregnation ratio, which is defined as a ratio (percentage) of the capacitance when a semiconductor layer is formed, assuming that the capacitance appearing when an electrolytic solution is impregnated in place of a semiconductor is 100%.
One of the methods for forming a semiconductor layer on the dielectric layer is a method of forming a semiconductor layer by energization. For example, a method of forming a semiconductor layer comprising a metal oxide by passing a direct current (Japanese Patent No. 1,985,056), a method of obtaining a semiconductor layer comprising an electrically conducting compound by passing an alternating current (Japanese Patent No. 2,826,341), or a method of obtaining a semiconductor layer comprising an electrically conducting polymer by contacting a separately prepared external electrode on a previously-provided chemical polymerization layer and by passing a direct current (Japanese Patent No. 1,988,457).